This invention relates to a power amplifier device for radio frequency signals and, more particularly, to a power amplifier device comprising a feedforward distortion compensating circuit (abbreviated to FF circuit in the following).
For simultaneous transmission of a plurality of carriers, a radio communication apparatus, typically in a base station of mobile communication, usually comprises amplifier circuits, equal in number to the carriers. Outputs of the amplifier circuits are combined by an antenna duplexer. This system has problems in that the apparatus becomes bulky with an increase in the number of carriers and in that a transmission frequency depends on the duplexer and can not be readily changed.
These problems would at once be solved if a power amplifier device were available for simultaneous amplification (common amplification) of a multiplicity of carriers. The carriers will be referred to as a plurality of radio frequency signals of different frequencies signal for convenience of description. In the mobile communication in particular, it would be possible at the same time to get rid of frequency restriction by the duplexer and to efficiently use frequencies. A conventional power amplifier device, however, has a nonlinearity which inevitably results in inter-modulation distortions on carrying out simultaneous amplification and gives rise to a deterioration of other communications of the mobile communication.
In order to prevent this, a power amplifier device must have a very high saturation output level. For example, let it be assumed that an output level per one carrier be 1 W, the carriers be ten in number, and the inter-modulation distortion should result in a spurious intensity of minus 60 dB. In order to subject the carriers to simultaneous amplification, the power amplifier device must have a saturation output of at least 1 kW and would be bulky. Practical implementation is consequently impossible with no improvement because both circuitry and power consumption are very uneconomical
A distortion compensating circuit has therefore been used in the power amplifier device. This is in order to reduce the saturation output level and to solve the problem of uneconomicalness.
Various power amplifier devices with distortion compensating circuits are already known. Among them, most excellent in view of a wide distortion compensating band, stability, and an amount of improvement in a distortion compensating effect is one that comprises an FF circuit.
Briefly speaking, a conventional power amplifier device comprises either a single FF circuit or a plurality of FF circuits in multiplicate. When the power amplifier device comprises the single FF circuit, the power amplifier device comprises a plurality of main amplifier circuits in parallel.
The conventional power amplifier devices and problems thereof will later be described in detail.